Switched Mode Power Supply Including a Flyback Converter with Primary Side Control

ABSTRACT

A method and apparatus for controlling a flyback converter are presented. The flyback converter includes a transformer, a semiconductor switch coupled to a primary winding of the transformer, a current measurement circuit coupled to the semiconductor switch, a diode coupled in series to a secondary winding of the transformer, and a controller. The controller is configured to receive a feedback voltage, a reference signal, and the measured primary current and generate a control signal for the semiconductor switch dependent on the feedback voltage, the reference signal, and the measured primary current. The semiconductor switch switches on and off cyclically in CCM operation.

This application is a continuation of Non-provisional application Ser. No. 13/857,642, filed on Apr. 5, 2013, which application is hereby incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a switched mode power supply including a flyback converter with a primary side control.

BACKGROUND

Switched-mode power supplies (SMPS) are commonly used and increasingly replacing “classical” power supplies composed of a transformer and a linear voltage regulator. SMPS use switching power converters to convert one voltage (e.g., an AC line voltage or a 13.8 V battery voltage) into another voltage, which may be used as supply voltage for an electric device or an electronic circuit. Many different switching power converter topologies are known in the field, such as buck converters, boost converters, Ĉuk converters, flyback converters, etc.

For safety reasons, it is desirable for the output of the power converter circuit to include galvanic isolation from the input circuit (connected to the utility power grid). The isolation averts possible current draw from the input source in the event of a short circuit on the output and may be a design requirement in many applications. Usually, optocouplers are used to galvanically isolate a feedback signal representing the regulated output voltage from the input circuit of the power converter circuit. The power conversion is accomplished by using a transformer. Transmitting feedback signals via optocouplers to ensure galvanic isolation often entails a comparably complicated feedback circuit.

Another design goal for the power conversion from the incoming AC line power to the regulated DC output current may be accomplished through a single conversion step controlled by one switching power semiconductor. A one-step conversion maximizes circuit efficiency, reduces cost, and raises overall reliability. Switching power conversion in the circuit design is necessary but not sufficient to satisfy the one-step conversion requirement while capitalizing on the inherent efficiency.

There is a need for a SMPS circuit that provides a regulated output voltage while not requiring any feedback signals to be tapped at the voltage output. Thus, optocouplers or similar components, which are usually employed for transmitting the current feedback signal back to the input circuit while providing a galvanic isolation, can be disposed of.

SUMMARY OF THE INVENTION

A method for controlling a flyback converter is described. The flyback converter may include a transformer that has a primary winding, a secondary winding, and an auxiliary winding, wherein the primary winding is operably carrying a primary current, the secondary winding is operably carrying a secondary current, and the auxiliary winding is operably providing a feedback voltage. The flyback converter may further include a semiconductor switch that is coupled in series to the primary winding for switching a primary current in accordance with a control signal, a current measurement circuit that is coupled to the semiconductor switch or the transformer for measuring the primary current, and a diode that is coupled in series to the secondary winding for rectifying the secondary current. Moreover, the flyback converter may include a controller for receiving the feedback voltage, a reference signal, and the measured primary current and is configured to generate the control signal for the semiconductor switch dependent from the feedback voltage, the reference signal, and the measured primary current. Thereby, the semiconductor switch switches on and off cyclically.

In accordance with a first aspect of the invention, the method comprises regularly interrupting the switching operation such that the secondary current drops to zero while the semiconductor switch is off, and sampling the feedback voltage at the time instant the secondary current reaches zero, thereby obtaining a first sampled value. The switching operation is continued and the feedback voltage is sampled at the time instant the control signal indicates a switching operation to switch on the semiconductor switch, thereby obtaining a second sampled value. Furthermore, measured primary current is sampled at the time instant the semiconductor switch has switched on, thereby obtaining a third sampled value. Finally, the reference signal is adjusted dependent on the first, the second, and the third sampled values.

Further, an electronic controller device for controlling the flyback converter is described. According to another aspect of the invention, the electronic controller device includes a controller that receives the feedback voltage, a reference signal, and the measured primary current and is configured to generate the control signal for the semiconductor switch dependent on the feedback voltage, the reference signal, and the measured primary current, wherein the semiconductor switch is switched on and off cyclically in CCM operation. The electronic controller device further includes a compensation circuit that receives the reference signal and is configured to regularly interrupt the switching operation such that the secondary current drops to zero while the semiconductor switch is off. The compensation circuit is further configured to sample the feedback voltage at the time instant the secondary current reaches zero, thereby obtaining a first sampled value. Furthermore, the compensation circuit is configured to resume the switching operation and to sample the feedback voltage at the time instant the control signal indicates to switch the semiconductor switch on, thereby obtaining a second sampled value. The compensation circuit is further configured to sample the measured primary current at the time instant the semiconductor switch has switched on, thereby obtaining a third sampled value. Moreover, the reference signal is adjusted dependent on the first, the second, and the third sampled values downstream of the controller.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, instead, emphasis is placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts. In the drawings:

FIG. 1 illustrates the basic structure of a switched mode power supply (SMPS) circuit arrangement using a flyback converter topology and including output voltage control;

FIG. 2 illustrates the example of FIG. 1 in more detail;

FIG. 3 is a timing diagram illustrating the operation of a flyback converter in general;

FIG. 4 is a timing diagram illustrating the operation of a flyback converter in accordance with one embodiment of the invention;

FIG. 5 illustrates a portion of the timing diagram of FIG. 4 in more detail;

FIG. 6 illustrates a flyback converter including a control unit in accordance with one example of the invention; and

FIG. 7 is a flowchart illustrating the method realized by the circuits of FIGS. 2 and 6.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 illustrates the basic structure of a switched mode power supply (SMPS) circuit arrangement in accordance to one example of the present invention. The circuit arrangement comprises, as a switching power converter, a flyback converter 1 which comprises a primary side and a secondary side which are galvanically isolated by a transformer having a primary winding L_(P) and a secondary winding L_(S) (see also FIG. 2). The primary winding L_(P) has N_(P) turns and the secondary winding has N_(S) turns.

The primary winding L_(P) of a flyback converter 1 is coupled to a rectifier 5 configured to rectify an alternating line voltage supplied by, for example, the power grid.

The secondary winding L_(S) of the flyback converter 1 is coupled to a load, i.e., the LED device 50, for supplying output power thereto. The flyback converter 1 further comprises a power semiconductor switch T₁ for controlling the current flow through the primary winding L_(P) (denoted as primary current i_(P)). That is, the semiconductor switch is configured to switch the primary current i_(P) on and off in accordance with a respective control signal. The circuit arrangement further comprises a current sense unit 15 (primary current sense) that provides a current sense signal V_(CS) representing the primary current i_(P) through the primary winding L_(P). The current sense unit 15 may include, for example, a shunt resistor (cf. resistor R_(CS) in FIG. 2) connected in series to the primary winding L_(P), and the voltage drop across that shunt resistor represents the primary current i_(P). The circuit arrangement further comprises a control unit 10 that generates the control signal V_(G) supplied to the semiconductor switch T₁ so as to switch it on and off in accordance with this control signal V_(G). The semiconductor switch T₁ may be, for example, a MOS field effect transistor (MOSFET). In this case the control signal V_(G) may be the gate voltage or the gate current applied to the MOS transistor.

Generally, control unit 10 controls the switching operation of the flyback converter 1. In the present example, the control unit 10 is configured to control the flyback converter 1 such that it operates in a quasi-resonant (i.e., self-oscillating) mode. The control unit 10 may further be configured to compare the current sense signal V_(CS) with a reference signal. The control signal V_(G), which controls the switching state of the semiconductor switch T₁, is set to switch the primary current i_(P) off when the primary current sense signal V_(CS) equals or exceeds the reference signal. In quasi-resonant mode the semiconductor switch T₁ is, for example, switched on when the voltage across the switch T₁ is at a (local) minimum. For this purpose, the circuit arrangement may comprise a voltage sense unit 13 for directly or indirectly monitoring the voltage drop across the semiconductor switch during the off-time of the switch in order to allow for detecting the time instant when the voltage is at the minimum. Thus, the switching losses and the electromagnetic emissions are minimized.

The circuit of FIG. 1 may be a multi-mode switching power converter with primary side (only) control. “Primary Side Control” means that the control unit 10 is capable of regulating the output voltage thereby using only (measured) signals available on the primary side of the transformer included in the flyback converter 1. Therefore, no signal has to be measured at the secondary side and transmitted (via a galvanic isolation) to the controller. An additional galvanic isolation (usually provided by optocouplers in known SMPS) in the feedback loop is thus not necessary. Multi-mode means that the control unit 10 is configured to operate in different modes such as CCM at high loads, quasi-resonant DCM at medium and low loads, and burst mode at very low loads. The present description, however, mainly deals with CCM operation of the control unit 10. Multi-mode control units for use in SMPS are as such known in the field and thus not further discussed herein.

FIG. 2 illustrates one exemplary implementation of the basic structure of FIG. 1 in more detail. The output voltage supplied to the load 50 is provided by a buffer capacitor C₁, output capacitor, or C_(OUT)) which is coupled parallel to a series circuit including the secondary winding L_(S) of the transformer and the flyback diode D₁. Energy is transferred from the primary side to the secondary side of the transformer in the time intervals during which the primary current i_(P) is switched off. During the same time interval, the buffer capacitor C₁ is charged via the flyback diode D₁ by the induced current flowing through the secondary winding L_(S).

The primary winding L_(P) is connected between an output of the rectifier 5 that provides the rectified line voltage V_(IN) and the semiconductor switch T₁ which controls the current flow (primary current i_(P)) through the primary winding L_(P). In the present example, the semiconductor switch T₁ is a MOSFET coupled between the primary winding L_(P) and the ground terminal providing ground potential GND1. A current sense resistor R_(CS) (also referred to as shunt resistor) may be connected between the source terminal of the MOSFET T₁ and the ground terminal such that the voltage drop V_(CS) across the current sense resistor R_(CS) represents the primary current i_(P). It should be noted that the current sense resistor R_(CS) is just one exemplary implementation of the current sense unit 15 illustrated in FIG. 1. Any other known current measurement method and related circuits are applicable as well. The voltage drop V_(CS) across the current sense resistor R_(CS) is provided as a current sense signal to the control unit 10 which generates a gate signal V_(G) supplied to the control terminal of the semiconductor switch (i.e., the gate electrode in the case of a MOSFET) for controlling the switching state thereof.

When the semiconductor switch T₁ is switched on, the primary current i_(P) starts to rise and the energy E stored in the primary winding L_(P) increases. Since the flyback diode D₁ is reverse biased during this phase of “charging” the inductance of the primary winding L_(P), the primary winding L_(P) behaves like a singular inductor and the energy E stored in the primary winding equals E=L_(P)·i_(P) ²/2. When the primary current i_(P) is switched off by the semiconductor switch T₁, the flyback diode D₁ becomes forward biased and the energy E is transferred to the secondary winding L_(S), whereby the secondary current i_(S) resulting from the voltage induced in the secondary winding L_(S) charges the output capacitor C_(OUT). The operating principle of the control unit 10, according to which time instants, is determined when the semiconductor switch T₁ switches on and off and will be explained later. However, the design of quasi-resonant flyback converters is well known in the art (see, e.g., Fairchild Semiconductor, “Design Guidelines for Quasi-Resonant Converters Using FSCQ-series Fairchild Power Switch,” in AN4146).

For detecting the time instances when to switch the primary current on, an auxiliary winding L_(AUX) (having N_(AUX) turns) may be magnetically coupled to the primary winding L_(P). A first terminal of the auxiliary winding L_(AUX) is coupled to the ground terminal GND1 whereas a second terminal of the auxiliary winding L_(AUX) may be coupled to the control unit 10 via a resistor R₁. Actually, a fraction of the voltage across the auxiliary winding L_(AUX) is supplied to the control circuit 10 via the voltage divider composed of the resistor R₁ and a further resistor R₂ coupled in series to resistor R₁. The fractional voltage is denoted as V_(FB) in FIG. 2. The series circuit of resistors R₁ and R₂ is coupled in parallel to the auxiliary winding L_(AUX) and the middle tap of the voltage divider is connected to the control unit 10. The resistors R₁ and R₂, together with the auxiliary winding L_(AUX), may be regarded as the voltage sense unit 13 illustrated in the general example of FIG. 1.

The auxiliary winding L_(AUX) may further be used for providing a supply voltage V_(CC) to the control unit 10 by means of a bootstrap supply circuit 12. When the primary current i_(P) is switched off, the voltage across the auxiliary winding L_(AUX) rises such that the bootstrap diode D₂ is forward-biased and thus allows for charging the bootstrap capacitor C₃. However, such bootstrap supply circuit is well known in present flyback converters and will not be further discussed here.

Flyback converters can be operated in continuous current mode (CCM, in which the secondary current does not drop to zero) and discontinuous current mode (DCM, in which the secondary current drops to zero and remains zero for a finite time). A special case of DCM is the limiting case between DCM and CCM (i.e., the transition between CCM and DCM) and sometimes referred to as critical conduction mode (CrCM, in which the secondary current drops to zero for only a very short time). The basic principles of controlling flyback converters in DCM and CCM are well known in the art and thus not explained here in more detail (see, e.g., J. I. Castillejo, M. García-Sanz, “Robust Control of an Ideal DCM/CCM Flyback Converter,” Proc. of the 10th Mediterranean Conference on Control and Automation (MED2002), Lisbon, Portugal, Jul. 9-12, 2002). In order to control the output voltage VT_(OUT) or the output current of the power converter, a respective feedback signal (representing the output voltage or current, respectively) may be fed back to the control unit 10. In order to provide a proper galvanic isolation, optocouplers are usually used in the feedback loop. To simplify the overall switched mode power supply (SMPS) circuit, so called “primary side control” concepts have been developed, according to which the output voltage to be regulated is estimated using measurements accomplished solely on the primary side of the flyback converter. Particularly, the secondary current i_(S) and the output voltage V_(OUT) may be observed (i.e., estimated) from the measured values of the primary current i_(P) and the feedback voltage V_(FB) obtained from the auxiliary winding L_(AUX).

FIG. 3 illustrates timing diagrams of the voltage V_(AUX) across the auxiliary winding L_(AUX) and the primary current i_(P) as well as the secondary current i_(S). The left diagram of FIG. 3 illustrates the SMPS operating in discontinuous current mode (DCM), whereas the right diagram of FIG. 3 illustrates the SMPS operating in continuous current mode (CCM). The timing diagrams are discussed in more detail below. In the left diagram of FIG. 3, the waveforms between the time instant t₁ and the time instant t₅ (when the semiconductor switch T₁ switches on after it has been switched off at time instant t₂) are continuously repeated during operation in DCM. At the time instant t₁, the semiconductor switch is switched on and the primary current i_(P) starts ramping up until a maximum current is reached at time instant t₂, when the semiconductor switch T₁ is switched off again. As a result, the primary current i_(P) quickly drops to zero, while the secondary current (almost immediately) rises to its maximum value and then ramping down until it reaches zero amperes at time instant t₄. When the semiconductor switch T₁ is in its on-state (i.e., switched on) between the time instants t₁ and t₂, the voltage V_(AUX) across the auxiliary winding is almost zero. When the semiconductor switch T₁ is switched off at time t₂ the voltage V_(AUX) steeply rises up to a maximum voltage. Some ringing of the voltage V_(AUX) may be observed between the time instants t₂ and t₃ (i.e., during a settling time), and between the time instants t₃ and t₄ (when the secondary current has dropped to zero) the voltage V_(AUX) drops to the value V_(OUT)N_(AUX)/N_(S), that is

V _(OUT)(t ₄)=V _(AUX) ·N _(S) /N _(AUX)(in DCM)  (1).

Equation (1) is valid for DCM only, in which the time instant t₄ is the time instant when the secondary current drops to zero. During the time interval between the time instants t₄ and t₅, the voltage V_(AUX) is ringing again and—when operating in quasiresonant mode—the semiconductor switch T₁ is switched on again when the voltage V_(AUX) reaches a (local) minimum, which is, in the present example, at time instant t₅. At the time t₅ the cycle starts over.

When operating in CCM, the situation is somewhat different as illustrated in the right diagram of FIG. 3. As the secondary current i_(S) never falls to zero, the forward voltage V_(D) across the flyback diode D₁, as well as the voltage V_(T) due to the (ohmic) resistance of the secondary winding L_(S), adds to the output voltage V_(OUT) in above-mentioned equation (1). That is, at time instant t₄ (when the semiconductor switch is switched on again) the voltage V_(AUX) can be equated as:

V _(AUX)(t ₄)=(V _(OUT) +V _(T) +V _(D))·N _(AUX) /N _(S)(in CCM)  (2).

The waveforms in the left diagram (DCM) and the right diagram (CCM) of FIG. 3 are essentially the same except that the semiconductor switch is switched on again before the secondary current i_(S) has dropped to zero.

So when (hypothetically) using equation (1) to calculate the output voltage V_(OUT) from the measured voltage V_(AUX), the difference between the actual output voltage V_(OUT) and the estimation V_(AUX)·N_(S)/N_(AUX) equates to (by combining equations (1) and (2)):

V _(AUX)(t ₄)·N _(S) /N _(AUX) −V _(OUT)(t ₄)=(V _(T) +V _(D))  (3)

which is equivalent to:

V _(AUX)(t ₄)−V _(OUT)(t ₄)·N _(AUX) /N _(S)=(V _(T) +V _(D))N _(AUX) /N _(S)  (4).

Equation (4) is valid not only at time instant t₄ but during the whole time interval between t₃ and t₄ in the right diagram of FIG. 3, which illustrates CCM operation. In this regard, it should be noted that the actual values of V_(T) and V_(D) may also be time-variant. As a consequence, a simple control unit which estimates the output voltage V_(OUT) in accordance with equation (1) would actually make the output voltage by V_(T)+V_(D) lower than expected. Moreover, the difference of equation (4) heavily depends on the actual output current as well as on temperature, diode characteristics, and PCB layout. Therefore, a precise output voltage regulation is difficult when using primary-side control.

One option would be to use a constant voltage offset so as to compensate for the mentioned (time-variant) offset V_(T)+V_(D). However, this solves the above-mentioned problem only for a specific diode characteristic, a specific PCB layout, and within a very narrow temperature range and output current range. Obviously, a more sophisticated approach would be highly useful.

As discussed above, the problem of insufficient proportionality between the voltage V_(AUX), which is observable at the auxiliary winding L_(AUX), and the actual output voltage V_(OUT) only occurs during continuous current mode (CCM, see right diagram of FIG. 3). Therefore, according to one aspect of the present invention, the control unit 10 is configured to insert one switching cycle (during CCM operation) in which the secondary current i_(S) is allowed to drop to zero before switching on the primary current i_(P). In other words, at least one DCM or CrCM switching cycle is inserted during CCM operation.

Inserting a single DCM switching cycle during CCM operation has only a negligible impact on the output voltage but allows for a precise measurement of the output voltage and current at the primary side of the flyback converter. The above-mentioned general concept is discussed in more detail below with reference to FIG. 4. Diagram (a) of FIG. 4 illustrates the insertion of a (longer) DCM cycle whereas diagram (b) illustrates a short DCM cycle of minimum length, i.e., a CrCM cycle. For further discussion, it should be noted that the time instants t₀ to t₅ in FIG. 3 do not correspond with the time instants t₀ to t₅ in FIG. 4.

Diagrams (a) and (b) of FIG. 4 are identical for times before the time instant t₅ when the secondary current i_(S) reaches zero. For times before the time instant t₄ both diagrams illustrated a switching operation in continuous current mode (CCM) as already illustrated in the right diagram of FIG. 3. Both diagrams of FIG. 4 illustrate the control signal V_(G) applied to semiconductor switch T₁ (top waveforms). In the present example, the control signal V_(G) is a gate voltage and semiconductor switch T₁ is a MOS transistors. Furthermore, the corresponding voltage V_(AUX) at the auxiliary winding L_(AUX) (middle waveforms) and the resulting primary and secondary currents i_(P) and i_(S) (bottom waveforms) are illustrated. At the time instant t₁ the gate voltage V_(G) switches from a low level to a high level thus activating the MOS transistor T₁ (i.e., switching it on). As a consequence, the secondary current i_(S) almost immediately drops to zero (as the flyback diode D₁ is now reverse biased) while the primary current i_(P) steeply rises to an initial value.

At time instant t₁′, the secondary current i_(S) is zero and the primary current i_(P) is at its initial value. After the time instant t₁′, the primary current further rises until it reaches its (pre-defined) maximum value at time instant t₂, at which the gate voltage V_(G) is reset, again, to a low level thus deactivating the MOS transistor T₁ (i.e., switching it off). During the time interval from t₁ to t₂ the voltage V_(AUX) is approximately −V_(IN)·N_(AUX)/N_(P). When the transistor T₁ is deactivated at time instant t₂, the primary current i_(P) almost immediately drops to zero (as the transistor T₁ is now in its blocking state) while the secondary current i_(S) steeply rises to an initial value.

At time instant t₂′, the primary current i_(P) is zero and the secondary current i_(S) is at its initial value. After the time instant t₂′ the secondary current almost constantly drops until it reaches its minimum value at time instant t₃, at which the gate voltage V_(G) is set, again, to a high level thus re-activating the MOS transistor T₁ in the same manner as in time instant t₁, and cycle starts over. During the time interval from t₂′ to t₃ the voltage V_(AUX) drops (after a short ringing during a settling time) from a maximum value to a minimum value V_(AUX)(t₃)=(V_(T)+V_(D)+V_(OUT))·N_(AUX)/N_(S) at time instant t₃ (cf. equation (2)), which is higher than the “ideal” value of V_(OUT)·N_(AUX)/N_(S) (cf. equation (1)). During CCM the switching frequency f_(SW) is fixed, so the time interval t₃-t₁ corresponds to the switching period f_(SW) ⁻¹.

Between the time instants t₃ and t₄, the signals have the same waveform as between the time instants t₁ and t₂ (while the semiconductor switch T₁ is on). At the time instant t₄ the semiconductor switch T₁ is switched off thus interrupting the primary current flow through the primary winding L_(P) and initiating the secondary current flow through the secondary winding L_(S). The primary current i_(P) drops to zero and the secondary i_(S) current rises steeply to its initial value between the time instants t₄ and t₄′ in the same manner as in the time interval between t₂ and t₂′. However, different from the previous period during which the semiconductor switch T1 was off, the secondary current is now allowed to drop continuously (at a substantially constant rate) until it reaches zero at time instant t₅. During the time interval from t₄′ to t₅, the voltage V_(AUX) drops (after a short ringing during a settling time) from a maximum value to a minimum value V_(AUX)(t₅)=(V_(OUT))·N_(AUX)/N_(S) at time instant t₅. As the secondary current i₅ is allowed to drop to zero, the cycle between t₃ to t₆ (i.e., when the semiconductor switch is reactivated) is a (single) DCM cycle while the previous cycles (e.g., between t₁ and t₃) and the subsequent cycles are CCM cycles, during which the secondary current i_(P) does not drop to zero. Starting from time instant t₆ the primary current i_(P) starts continuously increasing from zero to its maximum value and CCM operation is continued.

The only difference between diagram (a) and the diagram (b) of FIG. 4 is the duration of the time between t₅ and t₆ during which the primary and the secondary current i_(P) and i_(S), respectively, are zero. In this time period, the voltage V_(AUX) is oscillating and the switching time t₆ is chosen at a local minimum (also referred to as “valley point”) of the oscillation. When switching at such local minima, the flyback converter is operated in “quasi-resonant” mode. Quasi-resonant switching as such is known in the art (see, e.g., Infineon Technologies Asia Pacific, “Determine the Switching Frequency of Quasi-Resonant Flyback Converters Designed with ICE2QS01,” Application Note AN-ICE2QS01, Aug. 15, 2011) and thus not further discussed herein. From FIG. 4, one can see that when inserting a single DCM cycle during CCM operation, the voltage V_(AUX) is directly proportional to the output voltage V_(OUT) (the proportionality factor being the turn ratio N_(S)/N_(AUX)) at the time instant (t₅ in FIG. 4) when the secondary current reaches zero. Thus, inserting a DCM cycle allows for a precise output voltage measurement by monitoring the voltage V_(AUX) across the auxiliary winding L_(AUX) which is galvanically isolated from the secondary side without the need for an optocoupler.

From the diagrams in FIG. 4 and the equations (1) to (4) one can conclude that the offset voltage V_(T)+V_(D) due to the diode forward voltage V_(D) (of diode D₁, see FIG. 3) and the voltage V_(T) due to the overall line resistance of the secondary side equates to:

(V _(AUX)(t ₈)−V _(AUX)(t ₅))·N _(S) /N _(AUX) =V _(T) +V _(D) =V _(OFFSET)  (5)

wherein the time instant t₈ is the very moment when the control signal V_(G) (i.e., the gate voltage) changes from low to high, i.e., immediately before switching on the semiconductor switch T₁. It should be noted that V_(AUX)(t₈) may be measured in any CCM cycle, wherein V_(AUX)(t₁)=V_(AUX)(t₃)=V_(AUX)(t₈) as illustrated in FIG. 4 (both diagrams). The time instant t₅ is the very moment when the secondary current i_(S) falls to zero in a DCM cycle. It should be noted that V_(AUX)(t₅) may be measured in any DCM cycle, however, only a single DCM cycle is illustrated in each diagram of FIG. 4. From equation (5), it can be concluded that the offset voltage V_(OFFSET) is derivable from measurements of the voltage V_(AUX) at different time instants t₅ and t₈ wherein, as mentioned, t₅ represents any time instant during a DCM cycle when the secondary current reaches zero and t₈ represents any time instant during a CCM cycle immediately before the semiconductor switch T₁ is switched on. In order to determine the offset voltage V_(OFFSET) in accordance with equation (5), the problem of how to detect the time instant t₅ (i.e., when the secondary current i_(S) reaches zero) remains.

The time instant t₅ can be estimated as t₅=t₆-N_(ZCD)·π·sqrt(L_(p)·C_(T1ds)), where L_(p) is the transformer primary inductance, C_(T1ds) is the equivalent capacitance across T₁ drain and source pin, N_(ZCD) is the number of half-periods of the oscillation elapsed before the quasi-resonant switching zero-crossing point, e.g., N_(ZCD)=3 for FIG. 4 a, N_(ZCD)=1 for FIG. 4 b. N_(ZCD) will also be an odd number (see also FIG. 3). Quasi-resonant switching is known in the field (see, e.g., Infineon Technologies Asia Pacific, “Determine the Switching Frequency of Quasi-Resonant Flyback Converters Designed with ICE2QS01,” Application Note AN-ICE2QS01, Aug. 15, 2011) and thus not further discussed herein. In order to obtain the sample value V_(AUX)(t₅), the voltage V_(AUX) can be continuously sampled and stored between the time instants t₄ and t₆. Then, at time instant t₆ the sample value for time instant t₅ may be taken from the memory. Also a delay-line providing a delay of N_(ZCD)·π·sqrt(L_(p)·C_(T1ds)) could be used to delay the currently sampled value for the voltage V_(AUX), so that at time t₆ the sample value for the time instant t₅ is still available at the delay line output.

For the following it is assumed that the offset voltage V_(T)+V_(D) can be calculated as:

V _(OFFSET) =V _(T) +V _(D) =p·i _(S)(t ₈)  (6)

wherein the factor p may vary over time. This offset appears at auxiliary winding L_(AUX) as:

V _(COMP) =V _(OFFSET) ·N _(AUX) /N _(S)=(V _(T) +V _(D))·N _(AUX) /N _(S) =p·i _(S)(t ₈)·N _(AUX) /N _(S)  (7).

As the secondary current i_(S) is not directly measured (remember that measurements at the secondary side are to be avoided to maintain galvanic isolation), the secondary current has to be derived from primary current measurements which are accomplished using the current sense resistor R_(CS) (see FIG. 2) or, generally, the primary current sense unit 15 (see FIG. 1). During CCM operation, the secondary current i_(S) can be derived from the primary current i_(P) using the following equation:

i _(S)(t ₈)=i _(P)(t ₈′)·N _(P) /N _(S)  (8)

wherein the time instant t₈ represents any time during a CCM cycle at which the semiconductor switch T₁ starts to switch on (i.e., the control signal V_(G) changes from a low to a high level), and the time instant t₈′ represents any time during a CCM cycle at which the resulting primary current i_(P) has risen to its initial value. Combining equations (6) and (8) yields:

V _(OFFSET) =V _(T) +V _(D) =p·i _(S)(t ₈)=p·(N _(P) /N _(S))·i _(P)(t ₈′)=k·i _(P)(t ₈′)  (9).

That is, the offset voltage V_(T)+V_(D) can be calculated from the primary current at the time the semiconductor switch T₁ has switched on. This offset appears at auxiliary winding L_(AUX) as:

V _(COMP) =V _(OFFSET) ·N _(AUX) /N _(S)=(V _(T) +V _(D))·N _(AUX) /N _(S) =k·i _(P)(t ₈′)·N _(AUX) /N _(S)  (10).

The proportionality factor k may be regularly determined by measuring the voltage V_(AUX) at time instant t₈ in a CCM cycle and time instant t₅ in a DCM cycle (see FIG. 4). Combining equations (9) and (5) yields:

k=(V _(T) +V _(D))/i _(P)(t ₈′)=(V _(AUX)(t ₈)−V _(AUX)(t ₅))·(N _(S) /N _(AUX))/i _(P)(t ₈′)  (11).

To determine the proportionality factor k in accordance with equation (11), the primary current has to be sampled (measured) at a time instant immediately after the activation of the semiconductor switch T₁ (see FIG. 1). Oscillations of the primary current i_(P), which may occur right after switching on the semiconductor switch T₁, may deteriorate the measured current value. This situation is illustrated in FIG. 5. A current measurement at a time instant t_(A) (which corresponds to t₁, t₃, and t₈ in FIG. 4) is not reliable due to the oscillations. However, after the oscillations (which are a transient phenomenon) have decayed the primary current i_(P) rises linearly which allows an extrapolation of the primary current i_(P) (t_(A)) at time instant t_(A). Assuming that further current values i_(P)(t_(B)) and i_(P)(t_(C)) are sampled after the oscillations have decayed, then the current i_(P)(t_(A)) at time instant t_(A) can be calculated as

i _(P)(t _(A))=2·i _(P)(t _(B))−i _(P)(t _(C)) for t _(B)=(t _(A) +t _(C))/2  (12)

that is, the sampling time t_(B) is in the middle between the sampling times t_(A) and t_(C). This situation is illustrated in FIG. 5.

An exemplary circuit representing the internal design of the control unit 10 (see FIGS. 1 and 2), which controls the switching operation of the flyback converter, is illustrated in FIG. 6. The illustrated embodiment is able to achieve a tight output voltage regulation during CCM operation for a flyback converter that requires no feedback signal from the secondary side (i.e., primary side control). The control unit 10 receives, as feedback voltage V_(FB), a fraction R₂/(R₁+R₂) of the voltage V_(AUX) tapped at the auxiliary winding L_(AUX) and with a current sense signal V_(S) tapped at the current sense resistor R_(CS), which is coupled in series to the semiconductor switch T₁ and the primary winding L_(P). Furthermore, the control unit 10 is configured to provide a control signal V_(G) for the semiconductor switch T₁, i.e., a suitable gate voltage or gate current in the case of a MOSFET.

The control unit includes a voltage mode or a current mode controller 101 that is supplied with the signal V_(S) representing the primary current i_(P), the feedback voltage V_(FB), and a (corrected) reference voltage V_(CREF). The current mode controller 101 is configured to generate a binary signal from these input signals V_(S), V_(FB), and V_(CREF), wherein the binary signal is transformed in a control signal V_(G) that is suitable for switching the semiconductor switch T₁ on and off. The design and the operation of the voltage mode or a current mode controller 101 is as such known in the art and not further discussed here.

The corrected reference signal V_(CREF) is derived from a reference signal V_(REF) (which may be a constant voltage) which can be regarded as set point for the output voltage control. In order to compensate for the systematic error when measuring the output voltage V_(OUT) in accordance with equation (1) in continuous current mode (CCM), the reference signal V_(REF) is “corrected” by adding an offset in accordance with equation (10). That is, the corrected (adjusted) reference signal V_(CREF) can be determined in accordance with the following equation:

V _(CREF) =V _(REF) +V _(COMP) =V _(REF) +k·i _(p)(t ₈′)·N _(AUX) /N _(S)  (13)

wherein the factor k is determined in accordance with equation (11) during the inserted DCM cycles as discussed above with reference to FIG. 4. The time instants t₅, t₈, and t₈′ are those illustrated in FIG. 4 wherein t₅ represents the time instant in any DCM cycle in which the secondary current reaches zero, t₈ represents the time instant in any CCM cycle in which the semiconductor switch T₁ begins to switch on the primary current i_(P), and t₈′ represents the time instant in any CCM cycle in which the semiconductor switch T₁ has finished the switching process of switching on the primary current i_(P). The time instant t₈ can be detected as the time instant the control signal V_(G) changes from a low to a high level (i.e., at a rising edge of the gate voltage V_(G)), whereas the time instant t₈′ can be detected as the time instant the voltage V_(AUX) (and thus the feedback voltage V_(FB)) drops to −V_(IN)·N_(AUX)/N_(P) (i.e., at a falling edge of the feedback voltage V_(FB)).

A timer circuit 102 (timer) coordinates the insertion of DCM cycles during CCM operation and the sampling of the voltage V_(AUX) and the primary current i_(P) (i.e., of the measurement signal V_(S)) at different times. Moreover, the timer 102 triggers the insertion of DCM cycles and controls four switches S₁, S₂, S₃, and S₄. The feedback signal is connected to the current mode controller 101 via switch S₄. The switch S₃ allows the sampling of the feedback voltage V_(FB) (which is a scaled version of voltage V_(AUX)) at time instant t₅ within a DCM cycle. The switch S₂ allows the sampling of the feedback voltage V_(FB) at time instant t₈ within each CCM cycle, and the switch S₁ allows the sampling of the primary current (i.e., of the current sense signal V_(S)). The scaling factor R₂/(R₁+R₂) of the feedback voltage may be considered in the subsequent signal processing. In fact, the voltage divider R₁, R₂ may be omitted so that V_(FB)=V_(AUX).

The switch S₄ is closed during CCM operation and is opened regularly (periodically or from time to time) which triggers the insertion of a DCM cycle, as the feedback voltage V_(FB) “seen” by the current mode controller 101 is zero when the switch S₄ is open. At time instants t₅, t₈, and t₈′ sampled values of V_(FB) and V_(S) are stored in the registers D3, D2, D1, which are coupled with the switches S₃, S₂, and S₁, respectively. As such, the switches S₁ to S₃ and the registers D₁ to D₃, operate as sample and hold circuits, wherein each sample and hold circuit is formed by a pair of switch and register, and the respective switches are controlled by the timer unit 102. In each DCM cycle, the arithmetic and logic unit (ALU) 103 takes the register values and calculates an updated value for the factor k in accordance with equation (11). Then, in each cycle, an updated value for the primary current i_(P) is sampled and the voltages V_(COMP) and V_(CREF) may be calculated in accordance with equation (13).

The ALU 103, the switches S₁ to S₃, the timer unit 102, and the registers D3, D2, D1 can be regarded as part of a compensation circuit which is configured to adjust the reference signal V_(REF) dependent on the first, the second, and the third sampled values stored in the registers D3, D2, D1. This adjustment is accomplished upstream of the current mode controller 101 so that the current mode controller 101 receives the adjusted reference signal V_(CREF).

Below, the function of the SMPS circuits described herein is summarized. It should be noted that this is not an exhaustive summary of important features. Instead, emphasis is put on the basic function of the device. Details have already been discussed above with respect to the circuit diagrams shown in FIGS. 1, 2, and 6 and the timing diagrams shown in FIGS. 3, 4, and 5.

The flyback converter circuit includes a transformer, which has a primary winding L_(P), a secondary winding L_(S) and an auxiliary winding L_(AUX). During operation, the primary winding L_(P) carries a primary current i_(P), the secondary winding carries a secondary current i_(S), and the auxiliary winding provides a feedback voltage V_(FB). The flyback converter circuit includes a semiconductor switch T₁, which is coupled in series to the primary winding for switching a primary current i_(P) on and off in accordance with a control signal V_(G). A current measurement circuit is coupled to the semiconductor switch T₁ or the transformer for measuring the primary current i_(P) (current sense signal V_(CS)), and a diode D₁ is coupled in series to the secondary winding L_(S) for rectifying the secondary current i_(S). Moreover, the flyback converter circuit includes a control unit 10, that receives the feedback voltage V_(FB), a reference signal V_(CREF), and the measured primary current V_(CS). Generally, the control unit 10 is configured to generate the control signal V_(G) for the semiconductor switch T₁ dependent (only) on the feedback voltage V_(FB), the reference signal V_(CREF), and the measured primary current i_(P) (i.e., the current sense signal V_(CS)).

During CCM operation, the semiconductor switch T₁ is switched on and off cyclically (step 71 in FIG. 7). The method for controlling the flyback converter circuit includes (e.g., regularly, from time to time) interrupting the CCM switching operation such that the secondary current i_(S) is allowed to drop to zero while the semiconductor switch T_(S) is off (step 72 in FIG. 7). This step also could be regarded as “inserting” a single DCM switching cycle. Then, the feedback voltage V_(FB) is sampled (first sampled value) at the time instant (see FIG. 4, time instant t₅) the secondary current i_(S) reaches zero (step 73 in FIG. 7). The switching operation is then resumed. For example, the semiconductor switch T₁ may be switched on when the voltage drop across the switch T₁ reaches a minimum. This is the quasi-resonant switch-on condition illustrated in FIG. 4 (see FIG. 4, time instant t₆). At this point, CCM operation is resumed (step 74 in FIG. 7). The method further includes sampling (second sampled value, step 75 in FIG. 7) the feedback voltage V_(FB) at the time instant (see FIG. 4, time instant t₈) the control signal V_(G) indicates to switch the semiconductor switch T₁ on (i.e., V_(G) changes from a low level to high level).

Moreover, the current sense signal V_(CS) is sampled (third sampled value, step 76 in FIG. 7) at the time instant (see FIG. 4, time instant t₈′) the semiconductor switch T₁ has switched on. The time instant t₈′ may be detected as the time instant the voltage V_(AUX) has reached the minimum level of −V_(IN)(N_(AUX)/N_(P)) as illustrated in FIG. 4 a, whereas the time instant t₈ may be detected as the time instant the drive signal V_(G) rises from a low level to a high level to switch the transistor T₁ on. Having obtained the three sampled values discussed above, the reference signal V_(CREF) is adjusted (step 77 in FIG. 7) dependent on the first, the second, and the third sampled values. This adjusting can also be seen in FIG. 6 where the externally supplied reference signal V_(REF) is superposed with a “compensation signal” V_(COMP) to obtain the compensated reference signal V_(CREF).

Although various exemplary embodiments of the invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. It will be obvious to those reasonably skilled in the art that other components performing the same functions may be suitably substituted. It should be mentioned that features explained with reference to a specific figure may be combined with features of other figures, even though not explicitly mentioned. Further, the methods of the invention may be achieved in either all software implementations, using the appropriate processor instructions, or in hybrid implementations that utilize a combination of hardware logic and software logic to achieve the same results. Such modifications to the inventive concept are intended to be covered by the appended claims. 

What is claimed is:
 1. A method for controlling a switching converter, which is operated in continuous conduction mode (CCM) to convert an input primary current passing through a primary winding into a secondary current passing through a secondary winding, wherein the method comprises: regularly interrupting a switching operation of the switching converter such that the secondary current drops to zero at a first time instant while the primary current is switched off; sampling a feedback voltage at or close to the first time instant to obtain a first sampled value, wherein the feedback voltage is generated at an auxiliary winding that is magnetically coupled to the primary and secondary winding; and resuming the switching operation in CCM.
 2. The method of claim 1, wherein, while operating in CCM, the method further comprises: sampling the feedback voltage at a second time instant shortly before the primary current is switched on to obtain a second sampled value.
 3. The method of claim 2, wherein, while operating in CCM, the method further comprises: sampling the primary current at a third time instant at which the primary current is switched on to obtain a third sampled value.
 4. The method of claim 3, wherein, while operating in CCM, the method further comprises: adjusting a reference signal, which is used to generate a control signal to switch the primary current on and off, dependent on the first sample value, the second sample value and the third sampled values.
 5. The method of claim 1 wherein the switching converter comprises: a transformer composed by the primary winding the secondary winding and the auxiliary winding, the primary winding operably carrying the primary current, the secondary winding operably carrying the secondary current, and the auxiliary winding operably providing the feedback voltage; a semiconductor switch coupled in series to the primary winding and configured to switch the primary current on and off in accordance with a control signal, a current measurement circuit coupled to the semiconductor switch or the transformer and configured to measure the primary current, a diode coupled in series with the secondary winding and configured to rectify the secondary current; and a controller coupled to receive the feedback voltage, a reference signal and the measured primary current and configured to generate the control signal for the semiconductor switch dependent on the feedback voltage, the reference signal, and the measured primary current, wherein the semiconductor switch switches on and off cyclically in CCM operation.
 6. The method of claim 1 wherein the first sampled value is representative of an output voltage of the switching converter.
 7. The method of claim 1, wherein resuming the operation of a semiconductor switch comprises: detecting when a voltage drop across the semiconductor switch assumes a minimum; and switching on the semiconductor switch at the time instant the voltage drop across the semiconductor switch assumes the minimum.
 8. The method of claim 1, wherein interrupting the switching operation comprises blanking a feedback signal received by the controller.
 9. The method of claim 8, wherein blanking the feedback signal includes using a semiconductor switch to interrupt a signal path of the feedback signal to the controller.
 10. The method of claim 1, wherein resuming the operation of a semiconductor switch comprises providing a feedback signal to the controller.
 11. The method of claim 1, wherein the controller is configured to generate a control signal for a semiconductor switch such that the switching converter operates in continuous conduction mode (CCM).
 12. The method of claim 1, wherein the switching converter is a flyback converter.
 13. A switched-mode power supply (SMPS) circuit comprising: a transformer having a primary winding, a secondary winding, and an auxiliary winding, the primary winding operably carrying a primary current, the secondary winding operably carrying a secondary current, and the auxiliary winding operably providing a feedback voltage; a semiconductor switch coupled in series to the primary winding for switching the primary current on and off in accordance with a control signal; a controller configured to switch the semiconductor switch on and off cyclically in continuous conduction mode (CCM) operation; and a compensation circuit configured to regularly interrupt operation of the semiconductor switch such that the secondary current drops to zero while the semiconductor switch is off, and sample the feedback voltage at a first time instant to obtain a first sampled value, wherein the first time instant occurs when the secondary current reaches zero; and resume the operation of a semiconductor switch in CCM.
 14. The SMPS circuit of claim 13, wherein the compensation circuit is further configured to sample the feedback voltage at a second time instant to obtain a second sampled value, wherein the second time instant occurs when the control signal indicates to switch the semiconductor switch on.
 15. The SMPS circuit of claim 14, wherein the compensation circuit is further configured to sample a measured primary current at a third time instant to obtain a third sampled value, wherein the third time instant occurs when the semiconductor switch has switched on.
 16. The SMPS circuit of claim 15, wherein the compensation circuit is further configured to adjust a reference signal dependent on the first, the second, and the third sampled values upstream of the controller.
 17. The SMPS circuit of claim 13, wherein the first sampled value is representative of an output voltage of the SMPS.
 18. The SMPS circuit of claim 13, wherein the SMPS is a flyback converter.
 19. The SMPS circuit of claim 13, further comprising a current measurement circuit coupled to the semiconductor switch or the transformer for measuring the primary current.
 20. The SMPS circuit of claim 13, further comprising a diode coupled in series to the secondary winding for rectifying the secondary current.
 21. The SMPS circuit of claim 13, wherein the controller is further configured to receive the feedback voltage, a reference signal, and a measured primary current and to generate the control signal for the semiconductor switch dependent on the feedback voltage, the reference signal, and the measured primary current.
 22. The SMPS circuit of claim 13, wherein, in resuming the operation of a semiconductor switch, the compensation circuit is configured to detect when a voltage drop across the semiconductor switch assumes a minimum value and switch on the semiconductor switch when the voltage drop across the semiconductor switch assumes the minimum value.
 23. The SMPS circuit of claim 13, wherein the compensation circuit is further configured to blank a feedback signal received by the controller to interrupt the operation of a semiconductor switch.
 24. The SMPS circuit of claim 23, wherein the compensation circuit comprises a semiconductor switch configured to interrupt a signal path of the feedback signal to the controller.
 25. The SMPS circuit of claim 24, wherein the controller is configured to generate the control signal for the semiconductor switch such that the SMPS circuit operates in CCM. 